The nn quasi-free nd breakup cross section: discrepancies to theory and implications on the 1S0 nn force

TL;DR

This paper investigates discrepancies between experimental quasi-free neutron-neutron cross section data from neutron-deuteron breakup and theoretical models, highlighting the need to adjust the nn effective range parameter for better agreement.

Contribution

It demonstrates that the nn quasi-free scattering cross section is highly sensitive to the effective range parameter and suggests a specific reduction to reconcile theory with data.

Findings

Discrepancies between data and theory are significant.

The nn QFS cross section is sensitive to the effective range parameter.

A 12% decrease in the nn effective range aligns theory with experimental data.

Abstract

Large discrepancies between quasi-free neutron-neutron (nn) cross section data from neutron-deuteron (nd) breakup and theoretical predictions based on standard nucleon-nucleon (NN) and three-nucleon (3N) forces are pointed out. The nn 1S0 interaction is shown to be dominant in that configuration and has to be increase to bring theory and data into agreement. Using the next-to-leading order (NLO) 1S0 interaction of chiral perturbation theory (chiPT) we demonstrate that the nn QFS cross section only slightly depends on changes of the nn scattering length but is very sensitive to variations of the effective range parameter. In order to account for the reported discrepancies one must decrease the nn effective range parameter by about 12 % from its value implied by 19charge symmetry and charge independence of nuclear forces.